The invention relates to opto-electronic components, and, more particularly, to liquid crystal nematic substances for the chromatic display of numbers, symbols and images.
It has been known for some time that chromatic liquid-crystalline mixtures can be used for the production of opto-electronic components and displays, based on the guest-host effect G. H. Heilmeier, L. A. Zanoni; Appl. Phys. Letters 13, 91 (1968). Mixtures which use the guest-host effect consist of nematic matrix substances, and one or more dyestuffs, which are oriented because of the anisotropy of the matrix. If the dyestuff has an anisotropy of the light absorption (dichroism), it is possible to simultaneously change the absorption characteristics due to the reorientation of the matrix in an electrical field. In general, such dyestuffs have a polarization level of which oscillates parallel to the longitudinal axis of the obling dyestuff molecule.
In nematic matrix substances with a positive dielectric anisotropy, this means that in the absence of an electrical field these substances are chromatic, while when contacting the field they become colorless. In displays this results in the observation of colorless numbers and symbols on a colored background, i.e., a so called "display with a negative contract," which in general is disadvantageous. A "display with a positive contrast" could be achieved by using nematic matrix substances having a negative dielectric anistropy. Suitable nematic matrix substances for this purpose, however, are not available.
A "display with positive contrast" may be made by using dyestuffs having a negative dichroism, as well, which are dissolved in nematic matrix substances with positive dielectric anisotrophy. Such dyestuffs also must exhibit good stability, particularly against light, and, as well, against thermal, chemical and electrical influences. They must be sufficiently soluble in nematic substances to provide a good chromatic contrast. It is already known that derivatives of tetrazine, which themselves have liquid crystalline characteristics, are dyestuffs with negative dichroism (see DD-Patents Nos. 137 117, 137,118, 137,242, 151,950; Fukui et al., 8, Internat. Liquid crystal Conf. Kyoto 1980, Abstract No. E-15 p). However previously known tetrazine derivatives exhibit an absorption maximum of 550 nm, only, i.e., a reddish-purple color, and this hue cannot be changed.
Accordingly, it is the object of the invention to provide liquid crystal substances for the guest-host effect with positive contrast with a pure red color, and which are particularly stable against light, and also against thermal, chemical and electrical effects.
Another object of the invention to provide chromatic liquid crystalline compounds which allow the preparation of mixtures and compositions for the guest-host effect having a positive contrast and pure red color.
Still another object herein is to provide a positive contrast opto-electronic display component comprising a dye having a negative dichroism and a nematic matrix substance having a positive dielectric anisotropy, which component exhibits a pure red color in display.